Liquid-gas contactors are used to transfer one or more soluble substances from one phase to another. Examples of conventional contactors are packed towers, plate columns and wetted wall columns. In these systems, gas absorption of one or more components from a gas stream is accomplished by dispersing the gas as bubbles in packed towers and plate columns in a countercurrent flow to a liquid stream. Absorption efficiency is controlled apart from solubility considerations by the relative rate of the flows and the effective surface area of the gas flow bubbles. In wetted wall contactors the gas stream flows past a downward flow of liquid on the inside wall of a vertical tube. Gas stripping is used to transfer a gas dissolved in liquid into a gas stream. Similar contactors are used for gas stripping.
Conventional contactors have several deficiencies. Primary among these is the fact that the individual gas and liquid flows cannot be varied independently over wide ranges. Tray columns are prone to such problems as weeping at low gas flows and flooding at high liquid flows. Packed towers can flood at high flow rates. The use of low liquid flow rates in a packed tower can lead to channeling and reduced effective surface area Excessive frothing or foam formation can lead to process inefficiency. Wetted wall contactors have inherently low mass transfer coefficients, and can flood at high gas flow rates. The development of membrane contactors has overcome these deficiencies.
Membrane contactors are devices through which two fluid phases flow separated by a membrane permeable to the gas being transferred. If a microporous membrane is being used, the preferred method relies on the non-wetting characteristic of the membrane material and the pore size to prevent liquid from intruding into the pores and filling them. Gas transfer then occurs through the gas filled pores to or from the liquid, depending on whether the process is absorption or stripping. If a non-porous membrane is used, gas transfer is controlled by the diffusion rate in the non-porous layer of the membrane. While other membrane geometries are available for this use, hollow fiber membranes are ideally suited as contactors.
A hollow fiber porous membrane is a tubular filament comprising an outer diameter, an inner diameter, with a porous wall thickness between them. The inner diameter defines the hollow portion of the fiber and is used to carry one of the fluids. For what is termed tube side contacting, the liquid phase flows through the hollow portion, sometimes called the lumen, and is maintained separate from the gas phase, which surrounds the fiber. In shell side contacting, the liquid phase surrounds the outer diameter and surface of the fibers and the gas phase flows through the lumen.
The outer or inner surface of a hollow fiber membrane can be skinned or unskinned. A skin is a thin dense surface layer integral with the substructure of the membrane. In skinned membranes, the major portion of resistance to flow through the membrane resides in the thin skin. The surface skin may contain pores leading to the continuous porous structure of the substructure, or may be a non-porous integral film-like surface. In porous skinned membranes, permeation occurs primarily by connective flow through the pores. Asymmetric refers to the uniformity of the pores size across the thickness of the membrane; for hollow fibers, this is the porous wall of the fiber. Asymmetric membranes have a structure in which the pore size is a function of location through the cross-section, typically, gradually increasing in size in traversing from one surface to the opposing surface. Another manner of defining asymmetry is the ratio of pore sizes on one surface to those on the opposite surface.
Manufacturers produce membranes from a variety of materials, the most general class being synthetic polymers. An important class of synthetic polymers is thermoplastic polymers, which can be flowed and molded when heated and recover their original solid properties when cooled. As the conditions of the application to which the membrane is being used become more severe, the materials that can be used become limited. For example, the organic solvent-based solutions used for wafer coating in the microelectronics industry will dissolve or swell and weaken most common polymeric membranes. The high temperature stripping baths in the same industry consist of highly acid and oxidative compounds, which will destroy membranes made of common polymers. Perfluorinated thermoplastic polymers such as poly(tetrafluoroethylene-co-perfluoro(alkylvinylether)) (poly(PTFE-CO-PFVAE)) or poly(tetrafluoro-ethylene-co-hexafluoropropylene) (FEP) are not adversely affected by severe conditions of use, so that membranes made from these polymers would have a decided advantage over ultrafiltration membranes made from less chemically and thermally stable polymers. These thermoplastic polymers have advantages over poly(tetrafluoroethylene) (PTFE), which is not a thermoplastic, in that they can be molded or shaped in standard type processes, such as extrusion. Perfluorinated thermoplastic hollow fiber membranes can be produced at smaller diameters than possible with PTFE. Fibers with smaller diameters, for example, in the range of about 350 micron outer diameter to about 1450 micron outer diameter, can be fabricated into contactors having high membrane surface area to contactor volume ratios. This attribute is useful for producing compact equipment, which are useful in applications where space is at a premium, such as in semiconductor manufacturing plants.
Being chemically inert, the Poly(PTFE-CO-PFVAE) and FEP polymers are difficult to form into membranes using typical solution casting methods as they are difficult to dissolve in the normal solvents. They can be made into membranes using the Thermally Induced Phase Separation (TIPS) process. In one example of the TIPS process a polymer and organic liquid are mixed and heated in an extruder to a temperature at which the polymer dissolves. A membrane is shaped by extrusion through an extrusion die, and the extruded membrane is cooled to form a gel. During cooling the polymer solution temperature is reduced to below the upper critical solution temperature. This is the temperature at or below which two phases form from the homogeneous heated solution, one phase primarily polymer, the other primarily solvent. If done properly, the solvent rich phase forms a continuous interconnecting porosity. The solvent rich phase is then extracted and the membrane dried.
Hydrophobic microporous membranes are commonly used for contactor applications with an aqueous solution that does not wet the membrane. The solution flows on one side of the membrane and a gas mixture preferably at a lower pressure than the solution flows on the other. Pressures on each side of the membrane are maintained so that the liquid pressure does not overcome the critical pressure of the membrane, and so that the gas does not bubble into the liquid. Critical pressure, the pressure at which the solution will intrude into the pores, depends directly on the material used to make the membrane, inversely on the pore size of the membrane, and directly on the surface tension of the liquid in contact with the gas phase. Hollow fiber membranes are primarily used because of the ability to obtain a very high packing density with such devices.
Packing density relates to the amount of useful membrane surface per volume of the device. It is related to the number of fibers that can be potted in a finished contactor. Also, contactors may be operated with the feed contacting the inside or the outside surface, depending on which is more advantageous in the particular application. Typical applications for contacting membrane systems are to remove dissolved gases from liquids, degassing; or to add a gaseous substance to a liquid. For example, ozone is added to very pure water to form a solution used to wash semiconductor wafers. Many processing steps involved in chip manufacturing use very aggressive chemicals such as hot sulfuric acid, hydrogen peroxide, phosphoric acid, etc. for etching purpose. Since these chemicals are toxic and dangerous, transport, storage and proper disposal of these chemicals pose serious health and safety hazards to the workers in this industry. New processing technologies have been developed in the last few years by a number of chip tool manufacturers. Unlike the conventional process which uses many aggressive chemicals, the new processing technology utilizes only two chemicals—ozonated DI water and HF water. It has been demonstrated that almost all existing processing bath can be replaced using only these two chemicals.
While HF water can easily be produced, a good source for ozonated water has been a challenge. Although ozonated water is being used in chip plants today, most uses are for cleaning operations where only a couple of parts per million (ppm) of ozone concentration are needed in the fluid stream. However, to replace aggressive etch baths, much higher ozone concentration is needed. In general, the concentration ranges between 10-80 ppm. The water flow rate ranged between 5-40 liters per minute (lpm). Typical requirement is about 15 ppm at 20 lpm.
Prior to the present invention, a tube side contacting device has been provided wherein the hollow fibers are formed of a polyfluorinated alkoxyvinylether (PFA) polymer. This device is characterized by undesirably limited gas mass transfer through the hollow fibers. In addition, contact devices have been provided wherein the hollow filter membranes or spirally pleated membranes are formed of PTFE.
Ohmi et al., J. Electrochem. Soc., Vol. 140, No. 3, March 1993, pp. 804-810, describe cleaning organic impurities form silicon wafers at room temperature with ozone-injected ultrapure water. U.S. Pat. No. 5,464,480 shows that ozone diffused through a subambient temperature deionized water will quickly and effectively remove organic materials such as photoresist from waters without the uses of other chemicals. It is believed that lowering the temperature of the solution enables a sufficiently high ozone concentration in solution to substantially oxidize all of the organic material on the wafer to insoluble gases. The means for diffusing a gas can be any means which provides fine bubbles of ozone or other gases into the tank and uniformly distributes the gas throughout the tank.
In U.S. Pat. No. 5,464,480, preferably, the bubbles that are provided by the diffuser are initially about 25 to about 40 microns in diameter. The gas diffusers preferably are initially about 25 to about 40 microns in diameter. The gas diffusers preferably are made from a mixture of polytetrafluoroethylene (PTFE) and perfluoroalkoxylvinylether. By varying the temperature and pressure under which the mixture is prepared by methods known in the art, both porous and nonporous members are formed. The impermeable and permeable members are preferably comprised of about 95% PTFE and about 5% perfluoroalkoxylvinylether. The permeable member and the impermeable member may be joined by any number of methods as long as the result is a composite member that will not come apart under the stresses in the tank. Preferably, the members are heat sealed together, essentially melting or fusing the members together using carbon-carbon bonds. Once the permeable member is formed, a trench is bored out of the PTFE in the top portion of the member. The resulting diffuser has on the order of about 100,000 pores of a size of about 25 to about 40 microns in diameter through which gas may permeate into the treatment tank. The use of the trench in the diffuser allows the gas to diffuse into the tank as very fine bubbles. In applications for the semiconductor manufacturing industry, a device that supplied homogeneous bubble free ozone dissolved in ultrapure water would provide more efficient oxidation reactions because the reaction would not be localized at the bubbles. The more homogeneous solution would provide for a more uniform cleaning reaction. Furthermore, the high surface area to volume ratio inherent in hollow fiber devices would give a compact system, suitable for semiconductor operations.
Dissolved oxygen in ultrapure water is another problem in semiconductor device manufacturing. Oxygen removal to less than one part per billion (ppb) is required to prevent uncontrolled oxide growth. Potential problems associated with uncontrolled oxide growth are prevention of low temperature epitaxy growth, reduction of precise control of gate-oxide films, and increased contact resistance for VIA holes. This uncontrolled growth can be overcome by stripping dissolved oxygen to less than 1 ppb from the ultrapure water used in the manufacturing process. The high packing density and cleanliness associated with an all perfluorinated thermoplastic contactor are advantages in such applications.
U.S. Pat. No. 5,670,094 provides an oxidized water producing method in which a pressurized ozone gas is generated by an electric discharge type ozonator is dissolved in water to be treated through a hollow fiber membrane, characterized in that the water pressure inside the membrane is maintained higher than the pressure of the ozone gas supplied to the outside of the hollow fiber membrane to prevent tiny bubbles and impurities from getting mixed into the water being treated, and the ozone concentration in the treated water is controlled on the basis of the concentration of the ozone gas. This reference discloses only PTFE membranes and does not contemplate the use of an all perfluorinated thermoplastic contactor.
Commercially all available PTFE hollow tube contactors are referred to as “hollow tubes”, probably because they are relatively large. Patent PJ7213880A discloses the fiber manufacturing process for making composite PTFE hollow tubes for ozonizing applications. The first step of this process involves extruding PTFE paste derived from a mixture of PTFE powder and lubricants. After the tube is formed, the lubricants are extracted and the powder sintered into a slightly porous PTFE solid tube. The tube is then stretched longitudinally to make it porous. This is different than typical PTFE sheet membranes made by a similar process. To generate very fine microporous structures, characterized by a node to fibrils network, most PTFE membranes are made by biaxial stretching. For hollow fibers, the equivalent process would have been stretching the fiber radially. Probably because of the impracticality of such a step, this radial stretching step is missing from the disclosed process. Consequently, the pores in this tube are only “half-formed”, i.e., it did not attain the “node to fibril network” of flat sheet membrane. To compensate for this deficiency, the tube underwent a second step of laminating a regular microporous flat sheet membrane on top of the external surface of the porous tube. This step involves lamination of a long narrow strip of PTFE microporous membrane spirally on the surface of the tubing. This is a tedious, labor intensive process. Also, with the membrane laminated to the outside of the hollow tube, the resistance to mass transfer in tube-side flow could be higher in cases were the fluid partially intrudes into the support layer. This arrangement diminishes the potential of housing the membrane as the barrier for separating the two fluid phases. These deficiencies are overcome with the hollow fiber membranes of the present invention.
An advantage for contacting applications is that the very low surface tension of these perfluorinated polymers allows use with low surface tension liquids. For example, highly corrosive developers used in the semiconductor manufacturing industry may contain surface tension reducing additives, such as surfactants. These developers could not be degassed with typical microporous membranes because the liquid would intrude the pores at the pressures used and permeate, causing solution loss and excess evaporation. In addition, liquid filling the pores would greatly add to the mass transfer resistance of gas transport. U.S. Pat. No. 5,749,941 describes how conventional follow fiber membranes of polypropylene or polyethylene cannot be used in carbon dioxide or hydrogen sulfide absorption into aqueous solutions containing an organic solvent without the use of a solution additive to prevent leakage. While PTFE membranes would work in these applications, presumably because of their lower surface tension, they are difficult to process into hollow fibers. The membranes of the present invention are made from polymers having similar surface tension properties to PTFE and are more readily manufactured into small diameter hollow fiber membranes.
Accordingly, it would be desirable to provide a hollow fiber membrane contactor apparatus for forming a liquid solution from a gas and a liquid which provides high mass transfer rates of gas through the hollow fiber membranes. Such an apparatus can be formed of a suitably small size to permit its use with currently available apparatus for delivering a reagent to a conventional etching process for making electronic devices.